This paper presents a novel contribution to the purification of goat β-lactoglobulin by using an ultrafiltration membrane enzymic reactor. The basis of the purification process was the enzymic hydrolysis of contaminating proteins, α-lactalbumin and traces of serum albumin, by pepsin at 40 °C and pH 2, conditions under which β-lactoglobulin is resistant to peptic digestion. Simultaneously, β-lactoglobulin and peptides were separated by ultrafiltration. β-Lactoglobulin was retained in the reactor while peptides generated by hydrolysis from α-lactalbumin and serum albumin permeated through the membrane. The process was made continuous by the addition of fresh whey to replace the lost permeate. Three mineral membranes with 10, 30 and 50 kDa molecular mass cut-off were tested and the 30 kDa membrane was selected for the continuous process. The simultaneous purification and concentration of β-lactoglobulin from clarified goats' whey was achieved in a single step. The ultrafiltration membrane enzymic reactor could treat eight reactor volumes of clarified whey. The recovery of β-lactoglobulin was 74%, its purity was 84% and its concentration 6·6-fold that in the initial clarified whey.